1. Field of The Invention
The present invention relates to a light amplifying device to Raman-amplify a signal light and to a control method.
2. Description of The Related Art
The demand for optical communication apparatuses has been increasing due to an increase in communication traffic in recent years. As optical communication apparatuses are used in a local network or subscriber network, as well as in a backbone network, an optical communication system plays an important role in an information network in the world.
In the optical communication system, an optical amplifier repeated transmission system is used. In this system, an optical amplifier, such as EDFA (Erbium Doped Fiber Amplifier), that amplifies wavelength division multiplexed (WDM) light is provided for each transmission path. The optical amplifier enables long-distance transmission of large capacity with low cost and high reliability. When a transmission loss is high, due to a long transmission path or the like in the optical amplifier repeated transmission system, the power of signal components contained in a light input to the optical amplifier is low, and thus an SN (Signal/Noise) ratio can be degraded and a transmission characteristic can also be degraded.
As measures to avoid such a problem, distributed Raman amplification (DRA) utilizing a Raman effect can be used. In the DRA, excitation light is input to a transmission path so as to amplify a signal light propagating the transmission path, described in Japanese Unexamined Patent Application Publication Nos. 2001-7768, 2002-76482, 2004-287307, 2004-193640, or 2006-189465. Utilizing DRA, the power of signal components contained in a light input to a Raman amplifier increases and thus SN ratio and transmission characteristics are improved. Accordingly, a distributed Raman amplifier has already been in practical use as effective measures.
In the optical communication system, loss of a signal light in a transmission path is high when a repeating interval is long. A transmission loss in a typical transmission path is about 0.2 dB/km. The transmission loss becomes higher in accordance with a repeating distance. When various functional optical components are placed on the transmission path, a insertion loss of those functional optical components is added, further increasing the loss of the signal light. The power of the signal light is lower as a repeating loss is higher.
In optical amplifying apparatus used in optical communication systems, such as an EDFA or a Raman amplifier, noise light increases as the gain of the optical amplifying apparatus becomes higher. Therefore, in a transmission path with a high transmission loss, when a light amplifying apparatus with a high gain is used, the ratio of the power of signal components with respect to the power of noise light contained in a light is small. In the case where a signal light is WDM light, the ratio of the power of signal components with respect to the power of noise light contained in the light is smaller as the number of multiplexed signal lights is smaller.
Also, in the optical communication system, as well as the optical amplifying apparatus to amplify a signal light, a power control device is used to control the power of the signal light to be constant.
For example, a typical power control device supervises total power of a light, which is a sum of the power of the signal components and the power of the noise light, by using a branch coupler and a PD (photodiode), and controls a power of the signal component of each channel on the basis of information about the number of multiplexed signal lights. The information is received from a supervisory signal light transmitted in the optical transmission system.
In the Raman amplification, gain variations occur in respective wavelengths depending on a condition of the transmission path, the number of multiplexed signal lights, or a condition of excitation light. The gain variation causes power variations in respective multiplexed signal lights in output light of the Raman amplifier. The power variations in output light may be compensated by using an optical filter (GEQ: Gain EQualizer) having a fixed loss wavelength characteristic, such as described in above mentioned JP 2002-76482. Still, as the power variations in output light complexly change depending on conditions of a transmission path or the number of multiplexed signal lights, the power variations remain a disadvantage.
The conditions of a transmission path include an optical loss resulting from soil at a connecting portion of an optical connector to connect optical fibers or a bend loss of the optical fibers, manufacture variations in characteristic of a transmission path fiber (a loss coefficient, an effective cross-section area, and so on), loss variations due to fusion in the transmission path fiber, aging degradation, and outside air temperature. Power variations caused in output light are accumulated by an in-line amplifier in a subsequent stage, cause an increase in non-linear phenomena in a transmission path, degradation of the SN ratio, and excess of an allowable value of input power of a receiver, and degrade a transmission characteristic disadvantageously.
On the other hand, as described in above-mentioned JP2001-7768 or JP2002-76482, the power of excitation light can be controlled to obtain a uniform power wavelength characteristic by monitoring a power wavelength characteristic of output light by using a spectrum monitor, such as a spectrum analyzer. However, providing the spectrum monitor causes problems, such as a complicated structure of an optical circuit, complicated control, and an increase in cost and size.
Alternatively, power variations in output light caused depending on various conditions can be managed in a database, as described in above-mentioned JP2006-189465. However, as described above, various conditions of a transmission path change power variations in output light. Accordingly, an enormous amount of data, including all combinations of those conditions, is necessary to be held in order to compensate power variations in output light with high precision.
In addition, even if such an enormous amount of data is held, selecting an appropriate value therefrom requires time. Therefore, it is difficult to monitor and compensate power variations in output light caused by time-dependent factors, such as an optical loss resulting from a bend loss of optical fibers, loss variations due to fusion in transmission path fibers, aging degradation, and outside air temperature, in real time.